Acoustical drivers, and particularly low frequency drivers such as woofers, may be mounted in an enclosure. Two common types of driver enclosures are sealed enclosures (i.e., not open to a medium of transmission) and ported enclosures (i.e., open to a medium of transmission). The low frequency performance of driver mounted within a sealed enclosure is determined by the internal volume of the enclosure, while the low frequency performance of a driver mounted in a ported enclosure is determined both by the internal volume of the enclosure and the dimensions of the port.
In a rectangular loudspeaker enclosure designed to provide loading to a low frequency drive unit, a standing wave will occur at frequencies related to the interior liner dimensions (e.g, the height, width, length) of the enclosure. Specifically, standing waves will occur at frequencies corresponding to a wavelength equal to twice the linear dimension and multiples of that frequency. For example, if the width of an enclosure is W, standing waves having a wavelength equal to 2W, 2/3W, 2/5W, 2/7W, etc will occur in the enclosure. Standing waves can cause undesirable aberrations in the frequency response of the system. The lowest frequency standing wave occurs along the longest linear dimension (e.g., Jength) of an enclosure and will typically have the most noticeable negative effect on the performance of a loudspeaker.
To illustrate the problem of standing waves within a loudspeaker enclosure, consider the loudspeaker 10 shown in FIG. 1, which includes a ported rectangular enclosure 12 with a driver 14 mounted near one end of the enclosure 16. The internal length of the enclosure has a dimension equal to L. The lowest frequency standing wave will occur at a frequency corresponding with a wavelength equal to twice the effective length of the longest internal dimension of the enclosure (i.e., λsw1=2L). Such a standing wave will give rise to a pressure differential within the enclosure at the standing wave frequency, with a high pressure at one end of enclosure 12 and a high pressure at the other end of enclosure 12, where the pressure one end, e.g. end 18, end is out of phase with that of the other end, e.g., end 16. In other words, at a given moment in time, there will be a high negative pressure one end of enclosure 12 and a high positive pressure at the other end of enclosure 12.
In an actual enclosure configured as enclosure 12 depicted in FIG. 1 constructed using 0.5″ thick Medium Density Fiberboard (MDF) and having internal dimensions of 13.375″ long, 7.5″ wide, and 1.75″ high, the lowest frequency standing wave occurred at approximately 450 Hz and gave rise to the large aberration in the frequency response shown at point X on trace A in FIG. 2.
One approach to reducing the adverse effects of standing waves in the frequency response range of a loudspeaker is to include acoustically absorbent material (e.g., fiberglass) at one or more strategic locations within the enclosure. However, such an approach is highly dependent on where the material is located (which can be difficult to precisely determine) and the way in which material is packed. The present invention discloses another method of dealing with standing waves.